I suspect it is not a wind-up, but an attempt to try and force you out of the property. They are probably trying to do this because they realise they can make more money by redeveloping the cellar and can't because you are in the way.
In the first instance suggest talking to CAB on the grounds that it is cheaper, but at the end of the day you may need proper professional legal assistance.
The �700 is presumably for the ground rent and maintenance of external parts of the property. This sound like a shed-load of money every 6 months, but I could be in London I suppose. The quality of internal maintenance, provided you take reasonable steps to avoid water leaks (as your insurer's report appears to state) is your business.